Fannie and Freddie - too big to fail even though they already have failed and may fail again

Nearly a year and a half after the outbreak of the global economic crisis, many of the problems that contributed to it haven't yet been tamed. The U.S. has no system in place to tackle a failure of its largest financial institutions. Derivatives contracts of the kind that crippled American International Group Inc. still trade in the shadows. And investors remain heavily reliant on the same credit-ratings firms that gave AAA ratings to lousy mortgage securities.

Fannie and Freddie, for their part, remain at the core of a housing-finance system that inflated a dangerous housing bubble. After prices collapsed, sending shock waves around the world, the federal government put America's housing-finance system on life support. It has yet to decide how that troubled system should be rebuilt.

On Dec. 24, Treasury said there would be no limit to the taxpayer money it was willing to deploy over the next three years to keep the two companies afloat, doing away with the previous limit of $200 billion per company. So far, the government has handed the two companies a total of about $111 billion.

The government is willing to tolerate such open-ended exposure for two reasons. First, it sees the companies as essential cogs in the fragile housing market. Fannie and Freddie buy mortgages originated by others, holding some as investments and repackaging others for sale to investors as securities. Together with the Federal Housing Administration, they fund nine in 10 American mortgages. Worries about potential insolvency would cripple their ability to fund home loans, which would hamstring the market.